The present invention relates to a cathode ray tube, and in particular to a color cathode ray tube having an electron gun employing an internal voltage-dividing resistor.
Color cathode ray tubes used in TV receivers or information terminals, house an electron gun for emitting a plurality (usually three) of electron beams at one end of an evacuated envelope, a phosphor screen formed of phosphors coated on an inner surface of the evacuated envelope at the other end thereof for emitting light of a plurality (usually three) of colors, and a shadow mask which is closely spaced from the phosphor screen and serves as a color selection electrode. The electron beams emitted from the electron gun are deflected to scan the phosphor screen horizontally and vertically to form a rectangular raster by magnetic fields generated by a deflection yoke mounted externally of the evacuated envelope and display a desired image on the phosphor screen.
FIG. 12 is a cross-sectional view for explaining an exemplary configuration of a color cathode ray tube, and in FIG. 12. In this color cathode ray tube, an evacuated envelope is formed by a panel portion 1, a neck portion 2 and a funnel portion 3, and electron beams 16 emitted from an electron gun 9 housed in the neck portion 2 scan a phosphor screen 4 two-dimensionally by being subjected to horizontal and vertical deflection magnetic fields produced by a deflection yoke 10.
The electron beams 16 are modulated in amount by video signals supplied via stem pins 15, are color-selected by a shadow mask 5 disposed immediately in front of a phosphor screen 4, and impinge upon the phosphors of the corresponding primary colors to reproduce a desired color image. In FIG. 12, reference numeral 6 is a mask frame, 7 is a magnetic shield, 8 is a mask suspension mechanism, 11 is an internal conductive coating, 12 is a shield cup, 13 is a contact spring, and 14 is a getter.
Such cathode ray tubes employ a multistage focus lens system to obtain sufficiently small electron beam spots over the entire phosphor screen.
Japanese Patent Application Laid-open No. Hei 10-255682, for example, discloses an xe2x80x9cextended field lensxe2x80x9d serving as a main lens formed by disposing an intermediate electrode between an anode electrode and a focus electrode.
FIG. 13 is a schematic longitudinal cross-sectional view of an electron gun of a cathode ray tube disclosed in Japanese Patent Application Laid-open No. Hei 10-255682 and FIG. 14 is a cross-sectional view taken along line XIVxe2x80x94XIV of the electron gun shown in FIG. 13. The electron gun is of the extended field lens type comprising three equally spaced coplanar cathodes 309 (one for each electron beam), a first electrode 301, a second electrode 302, a third electrode 303, a fourth electrode 304, a 5-1st electrode (a focus electrode) 305, a 5-2nd electrode (a focus electrode) 306, an intermediate electrode 310, a sixth electrode (an anode electrode) 307 and a shield cup 308 arranged coaxially in the order named from the cathodes 309, and the cathodes and the electrodes are fixed in predetermined spaced relationship on a pair of glass beads 311.
A voltage-dividing resistor 312 fabricated on a ceramic substrate is housed within the cathode ray tube to obtain a voltage to be supplied to the intermediate electrode 310 within the cathode ray tube, and the voltage-dividing resistor 312 is fixed to one of the glass beads 311. A metal wire 314a surrounds the glass beads 311 and the voltage-dividing resistor 312 and is welded to the intermediate electrode 310 as shown in FIG. 14.
The electrons emitted from the cathodes 309 are focused by a prefocus lens formed by the cathodes 309, the first electrode 301, the second electrode 302 and the third electrode 303, next by a pre-main lens formed by the third electrode 303, the fourth electrode 304 and the 5-1st electrode 305, and then by a main lens formed by the 5-2nd electrode 306, the intermediate electrode 310 and the sixth electrode 307, onto a phosphor screen, and form an image on the viewing screen of the cathode ray tube.
The voltage applied to the intermediate electrode 310 is selected lower than an anode voltage, but higher than voltages applied to the focus electrodes by dividing the anode voltage using the voltage-dividing resistor 312. Provision of the intermediate electrode 310 forms a lens of the extended field type in which the potential distribution along the tube axis is made gentle from the anode electrode to the focus electrodes, reduces spherical aberration and consequently the diameter of the electron beam spots is reduced.
As shown in FIG. 14, the amount of electrical charges accumulated on the inner wall of a neck glass 317 is stabilized by attaching the metal wire 314a to the intermediate electrode 310 such that the metal wire 314a surrounds the glass bead 311 and the voltage-dividing resistor 312.
In the manufacture of a cathode ray tube, after the cathode ray tube has been exhausted of gases and sealed, so-called spot-knocking (high-voltage stabilization) of applying a high voltage of about twice the normal operating voltage for the cathode ray tube to its anode electrode is carried out to remove projections in electrodes of the electron gun or foreign particles within the cathode ray tube by forcing arcing between the electrodes and between the electrodes and the inner wall of the neck portion and to thereby prevent occurrence of arcing within the cathode ray tube during the normal operation of the completed cathode ray tube.
But, in a cathode ray tube employing the extended field lens formed by applying a voltage divided from the anode voltage using an internal voltage-dividing resistor to the intermediate electrode and the above-mentioned metal wire for suppression of discharge attached to and facing a focus electrode upstream of the intermediate electrode, when the spot-knocking of applying a high voltage of about 60 kV, for example, to the anode electrode is carried out with all the electrodes except for the anode electrode and the intermediate electrode being grounded, there has been a problem in that arcing occurs between the metal wire for suppression of discharge and the resistance element of the voltage-dividing resistor and consequently, an overcoat glass film covering a resistance element or an alumina ceramic substrate of the voltage-dividing resistor is often fractured, because the metal wire for suppression of discharge surrounding the voltage-dividing resistor is grounded and therefore a voltage difference of about 30 kV is produced between the metal wire and the resistance element.
It is an object of the present invention to provide a cathode ray tube incorporating an internal voltage-dividing resistor and having withstand voltage characteristics improved by heightening effects of spot-knocking sufficiently preventing fracture of the internal voltage-dividing resistor during the spot-knocking procedure.
A cathode ray tube in accordance with the present invention achieves the above object with the following representative configuration. A color cathode ray tube in accordance with the present invention is provided with a voltage-dividing resistor producing a voltage to be applied to one of focus electrodes of a focus lens for focusing an electron beam on a phosphor screen by dividing a voltage applied to an anode electrode and a metal conductor disposed to surround the voltage-dividing resistor for suppression of discharge. The voltage-dividing resistor comprises an overcoat insulating film, a resistance element and an insulating substrate stacked, and the resistance element comprises major resistance-forming regions disposed on opposite sides of the metal conductor where the resistance element extends meanderingly in a direction of a cathode ray tube axis, and another resistance-forming region containing a portion thereof facing the metal conductor where minimum distances L1 and L2 between the resistance element and two long sides of the insulating substrate extending in the direction of the cathode ray tube axis, respectively, are made larger than corresponding minimum distances between the resistance element and two long sides of said insulating substrate extending in the direction of the cathode ray tube axis in the major resistance-forming regions.